The present invention generally relates to a method and device for the failsafe monitoring of the rotational movement of a shaft.
More specifically, the invention relates to a method for the failsafe monitoring of the rotational movement of a shaft, comprising the steps of:
generating a characteristic pulse train with a number of pulses following one another at successive times, the time interval of which is dependent on the rotational movement of the shaft,
determining a monitoring time period,
monitoring whether an expected pulse of the pulse train occurs within the monitoring time period, and
generating a control signal if the expected pulse does not occur within the monitoring time period.
The invention further relates to a device for the failsafe monitoring of the rotational movement of a shaft, having a first part for picking up a characteristic pulse train with a number of pulses following one by another at successive times, the time interval of which is dependent on the rotational movement of the shaft, having a second part for determining a monitoring time period, having a third part for monitoring whether an expected pulse of the pulse train occurs within the monitoring time period, and having a fourth part for generating a control signal when the expected pulse does not occur within the monitoring time period.
Even more specifically, the invention relates to the failsafe monitoring of the connecting member between a rotational shaft of a press and a cam-operated switchgroup used for operational control of the press, which is sometimes called shear pin monitoring. However, the invention should not be restricted just to this specific application. On the other hand, it relates only to the failsafe monitoring of rotational movement. “Failsafe” means in this connection that the corresponding devices and equipment conform at least to category 3 of European Standard EN 954-1 or criteria generally recognized as being comparable.
The operation of a mechanical press is generally controlled by means of what is known as a cam-operated switchgroup. Such a switchgroup generates a number of successive pulses, the time interval of which is dependent on the rotational movement of the controlled or monitored shaft. The cam-operated switchgroup is connected to the shaft by a connecting member either directly via a transmission or indirectly via a chain or a toothed belt. In the case of a mechanical press, which is a source of considerable risk to the operating personnel during operation, the pulses of the cam-operated switchgroup must undergo failsafe monitoring to ensure that the press is operating properly. If the pulses do not occur, an immediate, failsafe shutdown of the press takes place, since otherwise an uncontrolled and consequently dangerous state exists.
The monitoring of the pulse train and the shutdown of the press take place with a failsafe monitoring device or a failsafe programmable controller in the aforementioned sense. This is because, on account of the safety-critical use, the devices have to meet especially high requirements in terms of their intrinsic failsafe nature. They are only authorized for safety-critical use by the responsible supervisory authorities, for example the employers' liability insurance associations, if conformity with the standards and criteria is completely verified.
For these reasons, previously known monitoring devices are of a relatively simple construction. They exclusively operate with selectable but predefined monitoring time periods. In the case of presses with a variable number of strokes, however, this leads to the problem that the monitoring time period must be set to the smallest number of strokes occurring and consequently to the slowest rotational movement of the shaft. Otherwise, small numbers of strokes give rise to false alarms, which lead to unnecessary and expensive downtimes. Greater numbers of strokes, however, produce relatively long reaction times when shutting down, which is disadvantageous in an emergency situation.